If you are working with databases, it is important for you to have a solid understanding of how data is organized and accessed in an efficient way. One of the important concepts in databases includes the use of keys in a database. These keys help you to ensure that the records are identified in a unique way and can be retrieved quickly. Among the various types of keys, the one key that often raises questions is the Alternate Key.
In this blog, we are going to walk you through everything you need to know about alternate keys in DBMS. So let’s get started!
Table of Contents:
What is a Key in DBMS?
In DBMS (Database Management System), a key is considered to be a specified field or a combination of fields that are used to identify each record in a table in a unique way. Keys are useful for maintaining data integrity and enabling efficient data retrieval. Without the use of keys, finding and referencing specific records would be slow and prone to errors. Among the most commonly used keys, the Primary Key is useful for identifying each row in a unique way. The Candidate Key works as a field that can serve as a unique identifier; the Alternate Key is a Candidate Key that is not chosen as the Primary Key; and the Foreign Key is responsible for linking records between different tables. Together, these keys build the foundation for organizing and managing relational data effectively.
What is an Alternate Key?
An alternate key is any key in a database that is responsible for identifying a row in a unique way, but is not chosen as the primary key. When there are multiple candidate keys in a table (all capable of identifying rows in a unique way), only one key is selected as the primary key, and the rest of them are considered as alternate keys. These alternate keys act as additional unique identifiers. They allow you to access data in the table in other ways, especially if it is not ideal or possible to use the primary key. Having alternate keys in a table adds flexibility and helps you to ensure that there are multiple reliable options for identifying records in a unique way.
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Characteristics of an Alternate Key
Some of the important characteristics of an Alternate Key are given below:
- Uniqueness
Just like a primary key, an alternate key should have unique values. This means that no two rows in the table can have the same value for the alternate key. It is important to have this uniqueness because it helps you to ensure that the alternate key can be used to identify and distinguish each record in the table in a correct way.
- Not-Null Constraint
An alternate key has the potential to become a primary key. Therefore, it must follow similar rules, one of them being that it cannot contain null values. It is compulsory for each record in the table to have a value in the alternate column. This is important because if there is a missing value (null), that record cannot be identified uniquely. This leads to problems with data accuracy and integrity in the database.
- Candidate Key
A candidate key is any field that can identify a row in a table in a unique way. An alternate key is one of those candidate keys that was not considered to be a primary key, but it has the qualities to be a primary key. If there are some issues with the current primary key and it does not meet certain requirements, you could use an alternate key instead. The choice depends on the design of the database and what fits best for that situation.
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How to Define Alternate Keys in SQL?
In SQL, an alternate key is not defined using a special keyword. It is created by applying the UNIQUE constraint to a column or a set of columns that contain unique values, but is not the primary key. This helps you to ensure that no duplicate values can be entered in that column, thus allowing it to serve as a backup unique identifier. Also, the term “alternate key” is not used directly in SQL; any column with the UNIQUE constraint, aside from the primary key, acts as the alternate key.
Here is an example given below for your reference.
Example:
-- Step 1: Create the table
CREATE TABLE Employees (
emp_id INT PRIMARY KEY,
email VARCHAR(100) UNIQUE,
phone VARCHAR(15) UNIQUE,
name VARCHAR(50)
);
-- Step 2: Insert sample data
INSERT INTO Employees (emp_id, email, phone, name) VALUES
(1, '[email protected]', '1234567890', 'emp1'),
(2, '[email protected]', '2345678901', 'emp2'),
(3, '[email protected]', '3456789012', 'emp3');
-- Step 3: Select from the table to generate output
SELECT * FROM Employees;
Output:
Explanation:
In the above table, emp_id is the primary key, while email and phone are alternate keys. This is because they are unique as well and can identify each employee individually.
Example of Alternate Key
An alternate key is basically a column (or combination of columns) that is used to identify a record in a table in a unique way. They are not selected as the primary key. It serves as a backup unique identifier. For example, in an employee table, both email and employee_id might be unique, but if you choose employee_id as the primary key, then email becomes the alternate key.
An example is given below for your reference.
Example:
-- Step 1: Create the table
CREATE TABLE Employees (
employee_id INT PRIMARY KEY,
email VARCHAR(100) UNIQUE,
name VARCHAR(50)
);
-- Step 2: Insert sample data
INSERT INTO Employees (employee_id, email, name) VALUES
(101, '[email protected]', 'emp1'),
(102, '[email protected]', 'emp2'),
(103, '[email protected]', 'emp3');
-- Step 3: Query the table to generate output
SELECT * FROM Employees;
Output:
Explanation:
In the above query, employee_id is the primary key, and email is the alternate key. This is because it is also unique, but not the main identifier.
Importance of Alternate Key
Given below are the key reasons why alternate keys are important.
- Data Retrieval Flexibility
Alternate keys provide you with alternate ways to find data in a table when the primary key is not available for a specific query. This can be helpful because you might need to search for records using various fields, and by having alternate keys, you can still get accurate results.
- Contribution to Normalization
In database normalization, where it helps you to reduce repeated data and avoid dependency issues, alternate keys can be useful. They help you to improve the structure and reliability of the database by giving extra ways so that you can access and connect to data without causing errors or duplication.
- Adapting to Business Logic
In real-life business scenarios, you need to identify records by using something other than the primary key. Alternate keys help you to do this by providing you with more options, which can help you to identify data in a unique way, based on the business needs or how the data is used.
Difference Between Primary Key and Alternate Key
The difference between Primary Key and Alternate Key is given below in tabular format.
| Feature |
Primary Key |
Alternate Key |
| Definition |
It is considered the main key that identifies rows in a unique way. |
It is a candidate key that is not chosen as the primary key. |
| Uniqueness |
A primary key must be unique. |
An alternate key must also be unique. |
| NULL Values |
Null values are not allowed in Primary Keys. |
Null values are also not allowed in Alternate Keys. |
| Number per Table |
You can only have 1 primary key per table. |
You can have multiple alternate keys per table. |
| Used in Relationships |
It is often used as a Foreign Key in other tables. |
It is rarely used in relationship cases. |
| Indexing |
A primary key is indexed automatically. |
Alternate keys may require manual indexing. |
| Selection |
A primary key is chosen among candidate keys. |
The keys other than the chosen candidate key is the alternate key. |
| Example |
Student_ID |
Email, Phone Number |
| Constraints Applied |
PRIMARY KEY |
UNIQUE |
Use Cases of Alternate Keys
Given below are some common scenarios where you can find the alternate keys to be valuable.
- User Authentication: A user might log in using his/her email or phone number. So both the email and the phone number must be unique to make sure that the system can identify the user correctly.
- Data Validation: Alternate keys help to prevent duplicate entries in fields like national ID numbers or tax identification numbers.
- Query Optimization: Alternate keys allow you to search or join tables using unique, indexing fields other than the primary key.
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Conclusion
Having a good understanding of alternate keys will help you design better and reliable databases. An alternate key works as a unique identifier, just like the primary key, but it is not chosen as the main identifier. It helps to strengthen data integrity, enhances the flexibility of querying, and makes the database more versatile.
By properly using alternate keys, you can ensure that your database is not only efficient but also resilient to issues regarding duplication of data. If you are building a relational database, you must always consider the fields that might serve as alternate keys, otherwise, you may face problems in the future.
What is an Alternate Key in DBMS – FAQs
Q1. Can a table have multiple alternate keys in a DBMS?
It is possible for a table to have one or more alternate keys. They could be chosen as primary keys, but they aren’t and still serve as unique identifiers for each record.
Q2. Do alternate keys automatically create indexes in all database systems?
Depending on the database you are using, not all alternate keys will form an index automatically. Although some databases build an index on columns marked with the UNIQUE constraint, others need to be manually indexed to run efficiently.
Q3. Can an alternate key be a composite key?
An alternate key is sometimes a composite key, too. As long as the grouping of the columns can uniquely identify each record, the right columns will be a composite primary key
Q4. Is it possible to change an alternate key to a primary key later?
Yes, it is possible. If your existing primary key is not an appropriate choice, a unique and non-null alternative can be changed to the primary key.
Q5. How do alternate keys help in preventing duplicate data entries?
Alternate keys ensure that a column has only one unique value with the UNIQUE constraint. As a result, the data quality remains good by preventing the same values from being entered more than once in these columns.